FIG. 1 shows rotor and sensors of a conventional SRM. A rotor 1 rotates on the generation of a magnetic field thereof by fixed magnetic poles. A sensor disc 2 is mounted on the same shaft with the rotor 1 to rotate together with the rotor 1. Position detection sensors S1, S2 and S3 detect the condition of the light passing or blocked by the sensor disc 2 on the rotation of the sensor disc 2. A logical circuit 5 generates phase excitation signals PH1, PH2 and PH3 on input of position detection signals from the position detection sensors S1, S2 and S3. The position detection sensors S1, S2 and S3 includes light emitter 3 emitting light and light receiver 4 receiving light respectively.
FIG. 2A shows a typical stator and the rotor of the SRM and FIG. 2B shows construction of a sensor disc of a conventional SRM.
As shown in FIG. 2B, the sensor disc 2 mounted on the rotor 1 and having projections 9 and cuts 10 rotates together with the rotor 1 when the rotor 1 rotates.
The light emitters 3 of the position detection sensors S1, S2 and S3 emit light on the rotation of the sensor disc 2. The light is passed or blocked by the projections 9 and the cuts 10 in the sensor disc 2. The light receiver 4 of the position detection sensors S1, S2 and S3 detect the continuous intermission of light. On supply of power to the SRM, the rotor 1 rotates and the sensor disc 2 mounted on the rotor 1 rotates. The emitted light is detected by the position detection sensors S1, S2 and S3 according to the rotation of the sensor disc 2. The light signals from the light emission are converted into a electrical signal.
The converted electrical signals are as shown in FIG. 3A.
In FIG. 3A, (a) is an output wave of the first position detection sensor S1, (b) is an output wave of the second position detection sensor S2 and (c) is an output wave of the third position detection sensor S3.
Referring to FIG. 2B, the output wave of the first position detection sensor S1 is explained. A low level signal is generated during the first 30 degrees because the light receiver 4 receives no light. The light receiver 4 receives light during next 60 degrees because the light is passed, thus the position detection sensor S1 generates a high level signal.
In this way the position detection sensor S1 generates the high or low level signal.
The signals as in FIG. 3A are supplied to the logical circuit 5 for generating phase excitation signals. Signals such as those shown as in FIG. 3B are generated from the logical circuit 5. These signals drive a gate of a transistor or a base of the transistor resulting in the rotation of the motor. Signals different than the signals shown in FIG. 3B can be generated by control signals (not shown) in the logical circuit 5.
FIG. 3B shows the rotor 1 rotating in counter clockwise direction. In this case, the power should be supplied to terminal A-A' first, to B-B' next and to C-C' third in FIG. 2A. But, in order to rotate the rotor 1 in clockwise direction, the power should be supplied to terminal A-A', and to C-C' and B-B' in sequence continuously thereafter.
FIG. 4 shows changes of inductance of a coil wound on a stator 6 according to change of phase. The inductance of the coil is the largest when there in no difference in phase ie, when the coil wound part of the stator 6 is matched with the projections 9 of the rotor, 1 and the smallest when the coil wound part of the stator 6 is positioned in 45 degrees by the phase difference with the projections 9 of the rotor 1.
Generally, excitation occurs in 45 degrees by the phase difference in the SRM, starting the excitation at the point of inductance increase of the coil. The motor will be braked, if the excitation is started at the point of inductance decrease. In another word, when the signals detected by the light receivers 4 are transmitted to the logical circuit 5, the logical circuit 5 generates the phase excitation signals PH1, PH2, and PH3 as shown in 3B and transmits these signals to terminals A-A', B-B' and C-C' shown in FIG. 2A, respectively to excite A phase winding La, B phase winding Lb and C phase winding Lc in sequence to rotate the rotor 1.
However, conventional SRM have the following problems. Various machining processes to mount the position detection sensors on the motors are required in order to detect the position of the rotor. Thus, the production processes are complicated. Space is required to mount the position detection sensors inside or outside of the motors, which makes the motors larger. Special sensors are required in case the motor should be operated in high temperature where ordinary sensors are not operative. Sensors like photo-interrupt element are unable to operate when the motors contain refrigerant or lubrication oil like compressors.